TARMAC BUILDING PRODUCTS LTD
FLOOD RISK ASSESSMENT
SLAB TRACK MANUFACTURING FACILITY, WESTBURY, WILTSHIRE
OCTOBER 2017
TARMAC BUILDING PRODUCTS LTD
FLOOD RISK ASSESSMENT
SLAB TRACK MANUFACTURING FACILITY WESTBURY, WILTSHIRE
OCTOBER 2017
172056A PMM GGN CD Issue 3 TBP FC elec 27-10-17 172056A PMM GGN CD Issue 2 TBP FC elec 26-10-17 172056A PMM GGN CD Final TBP FC elec 24-10-17 172056A PMM GGN CD Draft TBP FC elec 20-10-17 Job No Prepared Checked Approved Status Issued to No of Date by by by copies
FLOOD RISK ASSESSMENT SLAB TRACK MANUFACTURING FACILITY, WESTBURY, WILTSHIRE
CONTENTS PAGE NO.
1.0 INTRODUCTION 1
2.0 SITE PROFILE 2
3.0 EXTENT OF REDEVELOPMENT 7
4.0 TOPOGRAPHICAL SURVEY 9
5.0 APPROACH TO FLOOD RISK IN ENGLAND 10
6.0 ASSESSMENT FLOOD RISK 11
7.0 SEQUENTIAL AND EXCEPTION TEST 17
8.0 FLOOD MITIGATION MEASURES 21
9.0 CONCLUSION 26
APPENDICES Appendix A: Drawing LAF028/055 Site location Drawing LAF028/054 Site boundary Drawing LAF028/052 Development zones
Appendix B: Topographical survey layout Topographical survey sheets 1 to 7
Appendix C: Drawing 171056A-DC-GA-C-001 Historical flood mapping Drawing 171056A-DC-GA-C-101 Fluvial flood mapping Extract from West Wiltshire SFRA (2008) Drawing 171056A-DC-GA-C-102 Fluvial flood zone boundries Drawing 171056A-DC-GA-C-103 Pluvial flood mapping Drawing 171056A-DC-GA-C-104 Reservoir flood mapping Drawing 171056A-DC-GA-C-105 Hydrogeological mapping
Appendix D Greenfield run off rate estimation Brown field runoff rate estimation Storm Water Attenuation example
Flood Risk Assessment Slab Track Manufacturing Facility, Westbury, Co. Wiltshire
1.0 INTRODUCTION
1.1 Doran Consulting Ltd was commissioned by Tarmac Building Products Ltd (Tarmac) to undertake a Site Specific Flood Risk Assessment (FRA) Scoping Study for the proposed construction of a slab track manufacturing facility located at the Tarmac cement works located at Trowbridge Road, Westbury
1.2 This report has been prepared to accompany an outline planning application for the development. This report will review the available flood risk information published by the Environment Agency and assess the potential flood risk to the proposed development and the impact that any development may have on the flood risk to the surrounding area.
1.3 The report will, where possible, identify opportunities to reduce flood risk in the development area, and beyond, through the layout and form of the development, giving priority to sustainable drainage systems. The assessment will demonstrate that the proposed development can be considered to be appropriately flood resilient and resistant and that any residual risk will be safely managed.
1.4 The report will make reference to the historical operations conducted on the site throughout its operational lifecycle, which commenced in 1962, and that there has been no recorded instances of flooding on the site which has impacted or restricted operations on the site.
1.5 The general arrangement of the proposed development used within this report is an outline design and may be subject to change the during detailed design of the development.
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2.0 SITE PROFILE
2.1 Site Location
2.1.1 The site is located within the Westbury Cement Works, approximately 2 km northeast of Westbury Town, in the County of Wiltshire. Vehicular access to the site is via the Trowbridge Road. The site is connected to the national rail network via a private rail siding located on the southern boundary of the Westbury Cement Works.
2.1.2 The National Grid Reference for the proposed development site is 388710 152775.
Trowbridge
Site location A350
Figure 2.1: Site Location
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2.2 Site Description
2.2.1 The Westbury Cement Works was opened by Blue Circle Cement in 1962. Initially the site operated a single wet kiln to manufacture clinker through the heating of ground limestone and clay. The clinker was subsequently ground down into a fine powder to produce cement. The operation was expanded with the addition of a second wet kiln in 1965.
2.2.2 Throughout its lifetime the works produced approximately 650,000 tonnes of cement per year which was distributed by road and rail throughout the south and southwest of England. At its peak the Westbury Cement Works directly employed approximately 500 people.
2.2.3 In 2009 clinker and cement production at the site ceased however the current owners of the Westbury Cement Works, Tarmac, still operate a cement distribution business utilising a number of the large warehouses and cement silos located on the site. Currently the site directly employs approximately 20 people.
2.2.4 In 2016 an extensive demolition programme was undertaken in which all of the buildings and structures on the site except those used in Tarmac’s current cement distribution works were demolished. The buildings demolished included the two wet kilns, a 122m high chimney, a clinker grinding plant and numerous ancillary structures such as retention tanks and storage silos.
2.2.5 Figure 2.2 and 2.3 below provide aerial views of the site before and after the demolition works respectively.
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Figure 2.2 Westbury Cement Works prior to demolition works (viewing west)
Figure 2.3 Westburry cement Works post demolition works (viewing east)
2.2.6 The proposed slab track production facility is to be located within the area of the demolished works. This area includes the north, northeast and eastern sections of the Westbury Cement Works site.
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2.2.7 Located to the northeast of the site are three artificial ponds. These were originally used as balancing ponds during the operation period of the works however currently their only function is as part of the site storm and foul sewerage system.
2.2.8 A small private waste water treatment works is located in the north of the site. Foul sewerage generated from the Tarmac operations on the site is treated within the WWTW and is ultimately discharged into the ponds. Similarly storm water generated on the site is discharged into the ponds. Water from the ponds is discharged via two 150mm diameter pipes into a large clay pit located east of the site. Tarmac currently hold discharge consents issued by the Environment Agency permitting the discharge of 3m3 per day of foul sewerage into the ponds and discharge of 850m3 per day from the clay pit into the Heywood Brook located on the eastern boundary of the site.
2.2.9 The Heywood Brook rises approximately 2.5km east of the site near the village of Bratton. As the brook passes the Westbury Cement Works it forms the site’s eastern boundary. The Heywood Brook flows in a northwesterly direction to its confluence with the Bitham Brook approximately 1.5km north west of the site. The Bitham Brook in turn flows in a north westerly direction to its confluence with the Bliss Brook approximately 3km northwest of the site at Yarnbrook, where the two watercourses join forming the River Bliss.
2.2.10 Figure 2.4 below details the location of the watercourses in relation to the proposed development site.
River Bliss
Heywood Brook
Clay pit Bliss Brook Ponds
Bitham Brook
Site location
Figure 2.4 Watercourse near the development site.
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2.2.11 The catchment of the Heywood Brook measured from the furthest downstream extent of the site is approximately 6.3km2. The catchment area includes all of the Westbury Cement Works area, farmlands to the south and east of the cement works and the western part of Bratton village. Of the catchment area, approximately 0.25km2 (approx 4%) can be considered to be urban. This urban area is located within Bratton village at the extreme upper extent of the catchment. Figure 2.5 below, sourced from the Wallingford FEH online web service details the boundary of the watercourse catchment. (Note the catchment area has been verified by Doran Consulting against Ordnance Survey contour mapping.)
Site Location
Bratton Village
Figure 2.5 Wallingford FEH catchment boundary for watercourse.
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3.0 EXTENT OF REDEVELOPMENT
3.1 The proposed development is a concrete slab track manufacturing facility. It is expected that up to 85 full time staff will be employed when the facility is fully operational.
3.2 The facility will be located within the recently demolished northern and eastern sections of the Westbury Cement Works. The total area of the development site is approximately 12.2ha. Drawing LAF028/054 in Appendix A details the boundary of the development.
3.3 The proposed facility will consist of a large slab track manufacturing facility and supporting storage and logistics area where the formed precast concrete slab track units will be stored following production. The facility will utilise the existing rail connection to transport the finished precast units by train throughout the UK.
3.4 The development zone allocated for the proposed slab track manufacturing plant is approximately 4.5ha. This area is located immediately north and east of the existing Tarmac cement silos.
3.5 The storage and logistics zones are approximately 4.9ha in area. (Note this is exclusive of the manufacturing plant zone. The combined area of both zone types is therefore 9.4ha). The storage and logistics zone can be split into two distinct sections. The northern storage and logistics zone is approximately 2.5ha in area and is located immediately north of the proposed manufacturing plant zone. The eastern storage and logistics zone is approximately 2.4ha in area and is located east of the proposed manufacturing facility development zone and immediately north of the private rail siding zone.
3.6 The slab track manufacturing facility and supporting storage and logistic zones will be served by ancillary infrastructure such as an internal road network, rail mounted gantry cranes, drainage, services and lighting. The general arrangement of the manufacturing facility and of the ancillary infrastructure is to be confirmed during detail design.
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3.7 The private rail siding is located on the southern boundary of the Westbury Cement Works. This zone is approximately 2.8ha in area. Upgrade to the privately owned rail infrastructure within this zone may be undertaken as part of the proposed development works.
3.8 Drawing LAF028/052 in Appendix A presents the locations of the proposed precast slab track manufacturing facility development zone, storage and logistics zones and the private rail siding zone in relation to the existing structures within the Westbury Cement Works.
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4.0 TOPOGRAPHICAL SURVEY
4.1 A topographical survey of the Westbury Cement Works was undertaken in September 2017. The survey includes both the area used by Tarmac in their current cement distribution operation and the area of the proposed development site. The survey is presented within Appendix B.
4.2 Presently a large stockpile of clay (approx. 20,000m2) is located in the northern storage and logistics zone. This material will be relocated and reused on site as part of the re-development works. The topographical survey indicates the original ground level in this area ranges from approximately 61.5mOD at the northern boundary of the development area to approximately 64.0mOD in the west of this section. The clay stockpile varies in height up to a level of approximately 72.0mOD.
4.3 The slab track manufacturing facility will be located in the central section of the site. Presently within this area, rubble from the demolition works has been used as fill material to level the area and has also been cleared into a number of low profile stockpiles. The original ground levels in this section of the development site range between approximately 65.0mOD in the west of the section to 63.0mOD in the east.
4.4 In the eastern storage and logistics zone the ground profiles are formed from the intact floor slabs of the previously demolished buildings and the concrete service roads that served the buildings. Levels within this area range between approximately 62.75mO.D in the west of the section to 61.10mOD in the east.
4.5 Of note within the eastern storage and logistics zone is that there is a depression in ground levels in the approximate location of the demolished chimney. The ground level within the centre of this depression are approximately 60.90mOD. Review of the topographical survey does not indicate the presence of gullies within this depression, however there is the possibility that they were covered with material during the demolition works.
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5.0 APPROACH TO FLOOD RISK IN ENGLAND
5.1 This Flood Risk Assessment has been prepared in accordance with Chapter 10 of The National Planning Policy Framework (NPPF).
5.2 The NPPF outlines that a site specific flood risk assessment is required to assess the flood risk to a development site and demonstrate how flood risk from all sources of flooding to the development itself and the flood risk to others will be managed.
5.3 The site-specific flood risk assessment must demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall.
5.4 The National Planning Policy Guidance defines flooding in zones. The definitions of each zone are set out below:
Flood Zone 1 - land assessed as having a less than 1 in 1,000 annual probability of river or sea flooding (<0.1%)
Flood Zone 2 - land assessed as having between a 1 in 100 and 1 in 1,000 annual probability of river flooding (1% – 0.1%), or between a 1 in 200 and 1 in 1,000 annual probability of sea flooding (0.5% – 0.1%) in any year
Flood Zone 3 - land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%), or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year
5.5 Developments located within Flood Zones 1 and 2 are therefore considered to be at low risk to flooding during a 1 in 100 year flood event. Developments located within flood zone 3 are considered to be at risk to flooding during a 1 in 100 year flood event.
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6.0 ASSESSMENT FLOOD RISK
6.1 Introduction
6.1.1 This report has been prepared to accompany an outline planning application for the development. At this stage of the design process, a reasonable approach to assessing the potential risk of flooding to the development is through the review of the Environment Agency’s long term strategic flood mapping.
6.1.2 The Environment Agency flood maps are based on modelled data that utilises LIDAR data to determine geographical terrain levels on a catchment wide area. The flood zones identified on the Environment Agency’s maps are intended to be for strategic purposes only and as such are indicative and are not considered to be site specific.
6.1.3 For detailed site specific information, modelling of the Heywood Brook using topographical survey data of the watercourse, development site and surrounding area would be required to provide an accurate representation of the flood level and flood extents experienced during a 1 in 100 year flood event. It is recommended that this is undertaken during the detailed design stage of the development to inform the full planning application.
6.2 Historical Flooding
6.2.1 Construction of the Westbury Cement Works commenced in 1960 with the site becoming operation in 1962. The site has remained in continuous operation to present day.
6.2.2 During the 57 year operational history of the site there has been no recorded instances of flooding occurring within any areas of the Works. The enquiries, which have been undertaken by Tarmac, have included speaking to long- serving employees who are very familiar with the site and its history of operations.
6.2.3 Drawing 172056A-DC-GA-C-001 in Appendix C details the Environment Agency’s historical flood mapping in relation to the proposed development. Review of the mapping indicates that there have been no recorded instances of flooding at the site.
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6.3 Fluvial
6.3.1 Review of the Environment Agency long term fluvial flood risk mapping indicates that there may be a risk of fluvial flooding on the Heywood Brook. The West Wiltshire Level One Strategic Flood Risk Assessment Report (August 2008) (Appendix C) further details that the areas subject to flooding are classified as being within the Flood Zone 2 and Flood Zone 3. Drawing 172056A-DC-GA-C-101 in Appendix C details the extents of the Flood Zones in relation to the proposed development.
6.3.2 Drawing 172056-DC-GA-C-102 (appendix C) details that the proposed slab track manufacturing facility development zone is located within Flood Zone 1, approximately 25m west of the nearest Flood Zone 2 boundary and 35m west of the nearest Flood Zone 3 boundary.
6.3.3 The northern storage and logistics zone is located within Flood Zone 1 approximately 60m west of the nearest Flood Zone 2 and 3 boundary (boundary line is the same of both zones).
6.3.4 As both the slab track manufacturing facility development zone and the northern storage and logistics zone are located within Flood Zone 1 it can be considered that the risk of fluvial flooding during a 1 in 100 year flood event to these sections of the development are low.
6.3.5 Part of the eastern storage and logistics zone and a very small part of the rail siding zone are located within the Heywood Brook’s Flood Zone 2. The total area indicated within Flood Zone 2 is approximately 2,400m2. This equates to 10% of the eastern storage and logistics zone area and approximately 2% of the whole development area.
6.3.6 As Flood Zone 2 identifies areas that have between a 1 in 100 and 1 in 1,000 annual probability of river flooding, the risk of flooding to this section of the development during a 1 in 100 year flood event is considered to be low.
6.3.7 Part of the eastern storage and logistics zone and a very small part of the rail siding zone are located within the Heywood Brook’s Flood Zone 3. The total area indicated within Flood Zone 3 is approximately 8,600m2. This equates to 35% of the eastern storage and logistics zone area and approximately 7% of the whole development area.
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6.3.8 Overlay of the predicted Flood Zone 3 boundary extents onto the site’s topographical survey provides an estimate of the flood level experienced during a 1 in 100 year fluvial flood event. This method of analysis indicates that a flood level of approximately 61.25mO.D is experienced at this location during a 1 in 100 year flood event. (note this does not account for climate change).
6.3.9 The current topographical levels of the affected area range between 61.30mOD and 60.90mO.D. This would indicate that, during a 1 in 100 year flood event, the area of the eastern logistics zone within Flood Zone 3 experiences flooding to an estimated depth of up to 350mm.
6.3.10 As a section of the storage and eastern logistics zone and a small section of the rail siding are located within Flood Zone 3 it is considered that they are at risk to flooding during a 1 in 100 year flood event. The required extent of the logistics and storage area has only been developed in outline at this stage. Its maximum extent will be more accurately determined as part of the detailed design stage.
6.4 Pluvial
6.4.1 Review of the Environment Agency’s long-term flood mapping for medium risk pluvial flooding indicates that there are a number of small areas located within the development area that may be subject to pluvial flooding in a 1 in 100 year flood event. 172056-DC-GA-C-103 (Appendix C) details the extents of the pluvial flood risk areas in relation to the proposed development.
6.4.2 The proposed slab track manufacturing facility development zone is located outwith the predicted areas of pluvial flooding. As such, it is considered that the risk of pluvial flooding to this section of the development during a 1 in 100 year flood event is low.
6.4.3 A small area located on the eastern boundary of the storage and northern logistics zone is identified as at risk to pluvial flooding during a 1 in 100 year flood event. This area is approximately 430m2 with the depth of flooding estimated to range up to 300mm, increasing to above 300mm on the eastern boundary of the development site.
6.4.4 There are two small areas located within the eastern storage and logistics zone which are identified as at risk to pluvial flooding during a 1 in 100 year flood event. The first area is located south of the recently demolished wet kiln building. This area of flooding is approximately 330m2 with the depth of Flood Risk Assessment Slab Track Manufacturing Facility, Westbury, Co. Wiltshire 13
flooding estimated to be up to 300mm. The second area of pluvial flooding is located immediately west of the recently demolished chimney. This area is approximately 430m2 with the depth of flooding estimated to range up to 300mm increasing to above 300mm the centre of the area.
6.4.5 There are two areas located within the rail siding zone which are identified as at risk to pluvial flooding during a 1 in 100 year flood event. The first area is located south of the recently existing cement silos and is approximately 1,100m2 with the depth of flooding estimated to be up to 300mm, increasing to above 300mm adjacent to the silos. The second small area of pluvial flooding is located west of first is approximately 330m2 with the depth of flooding estimated to range up to 300mm.
6.5 Coastal
6.5.1 The development site is located approximately 46km from the nearest coast. Ground levels on the site are in excess of 60mOD. Due to the elevation of the site and its distance from the coast the risk of coastal flooding to the site is considered to be low.
6.6 Reservoir
6.6.1 A reservoir is located approximately 1km north of the site at Long Mead Wood. The reservoir discharges water on its southern boundary to a small watercourse that flows in a southwesterly direction to a confluence point with the Heywood Brook 800m northwest of the proposed development site.
6.6.2 The Environment Agency long term reservoir inundation flood risk map for the area indicates that in the event of an uncontrolled release of water from the reservoir the watercourses downstream of the reservoir will be subject to out of channel flooding to depths of up to 2m. Some sections of the watercourse immediately south of the reservoir may be subject to flooding to a depth of over 2m.
6.6.3 172056-DC-GA-C-104 (Appendix C) details the extents of the reservoir flood risk areas in relation to the proposed development. The predicted extent of reservoir inundation flooding terminates approximately 530m north of the boundary of the proposed development site. As the proposed development site is located upstream and outwith the predicted extent of flooding, the flood risk
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at the site due to an uncontrolled release of water from the reservoir is considered to be low.
6.7 Groundwater flooding
6.7.1 The British Geological Society geology records for the area of the proposed development site state that the underlying bedrock is a sedimentary mudstone formed approximately 150 million years ago. The mudstone is part of the Kimmeridge Clay Formation.
6.7.2 The records also indicate superficial deposits of Head material comprising of clay with silts, sand and gravels in the eastern part of the site. No records of the superficial deposits are held for the central and western sections of the site however it is reasonable to assumed that they are clays with silts, sand and gravels.
6.7.3 Review of the British Geological Society hydrogeology records for the site state that the bedrock is classified as mudstone with essentially no ground water. The risk to the site from ground water flooding generated from within the boundary of the site is therefore considered to be low. Drawing 172056-DC- GA-C-105 (Appendix C) details the hydrogeological zones in relation to the proposed development.
6.7.4 Within the boundary of the Heywood Brook catchment area, south of the development site, there are areas where the underlying bedrocks are formed from sandstone and chalk. The hydrological records classify the sandstone band as a moderately productive aquifer with an intergranular flow mechanism. The chalks further south are classified as a highly productive aquifer system with the flow through the chalk restricted to fractures and other discontinuities within the chalk.
6.7.5 Any ground water flooding generated in the area south of the site will enter the watercourse network upstream of the site. It is therefore considered that the risk of ground water flooding to the site generated from areas outwith the site boundary is low.
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6.8 Out of Sewer Flooding.
6.8.1 When fully operational the Westbury Cement works was served by a storm sewer network that discharged into the artificial storage ponds located in the north east of the site. Storm water generated on the site was attenuated within the ponds prior to its controlled discharge into the Heywood Brook.
6.8.2 Following the closure of the clay pit, water from the storage ponds has been redirected from the Heywood Brook into the clay pit. Water attenuated within the pit is discharged at a controlled rate into the Heywood Brook. As the water level within the pit is at a lower elevation that that of the Heywood Brook a discharge pump is utilised to lift the water.
6.8.3 In the central and western sections of the Westbury Cement works, where the current Tarmac cement distribution operations are located, the original storm and foul sewer network remains in operation. Tarmac currently hold a discharge consent issued by the Environment Agency for the discharge of 850m3 per day to the Heywood Brook.
6.8.4 In the northern and eastern sections of the Westbury Cement Works, where the proposed slab track manufacturing facility is to be located, it is assumed that following the demolition and clearance of buildings the storm and foul networks are no longer operational.
6.8.5 The proposed development of the slab track manufacturing plant and the storage and storage zones will be served by new storm and foul sewer networks designed to the appropriate current design standards. Storm water generated will be attenuated to a maximum agreed discharge rate. At this early stage of the design process, it is considered that existing arrangement of attenuation utilising the artificial ponds will be maintained.
6.8.6 There are no other known storm or foul sewer networks in the vicinity of the site. As the proposed development will be served by sewer networks designed to current standards it is considered that the risk of out of sewer flooding to the development site is low.
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7.0 SEQUENTIAL AND EXCEPTION TEST
7.1 Sequential Test
7.1.1 Paragraph 100 of the National Planning Policy Frame Work (NPPF) Planning Policy Guidance (PPG) recommends that the Sequential Test is to be undertaken if a development is indicated to be within Flood Zone 2 or 3. The aim of the Sequential Test is to direct development to areas with the lowest probability of flooding. PPG further states that a pragmatic approach on the availability of alternative sites should be taken during the assessment.
7.1.2 Table 2 of PPG classifies development in relation to flood risk vulnerability. Using this method of classification, buildings used for general industry are classified as Less Vulnerable. Outdoor storage and logistic areas such as that proposed in the development are not directly classified under Table 2 of PPG, however for the purpose of this report it is reasonable to assumed that they are classified as Less Vulnerable.
7.1.3 In the application of the Sequential Test reference is made to Table 7.1 below, reproduced from Table 3 of the PGG.
Flood Flood Risk Vulnerability Classification Zones Essential Highly More Less Water infrastructu Venerable Venerable Venerable Compatible re Zone Appropriate Appropriate Appropriate Appropriate Appropriate 1 Zone Appropriate Exception Appropriate Appropriate Appropriate 2 Test Required Zone Exception Should not Exception Exception Should not 3a Test Required be permitted Test Required Test Required be permitted Zone Exception Should not Should not Should not Should not 3b Test Required be permitted be permitted be permitted be permitted Table 7.1 PPG Flood Risk Vulnerability and Flood Zone Compatibility
7.1.4 Section 6 of this Report identifies that the majority (91%) of the proposed development is located within Flood Zone 1. The areas of the development within Flood Zone 1 includes the slab track manufacturing facility development zone, all of the northern storage and logistics zone and the majority of both eastern storage and logistics zone and the rail siding zone. Application of the
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Sequential Test to these areas of the development concludes that they are suitable for development.
7.1.5 Section 6 of the Report identifies that an area of approximately 2,400m2 (2% of development area) of the development is identified as within Flood Zone 2. Application of the Sequential Test to this section of the site concludes that it is suitable for development.
7.1.6 Section 6 of the Report identifies that an area of approximately 8,600m2 (7% of development area) of the development is identified as within Flood Zone 3. Application of the Sequential Test to this section of the site concludes that an Exception Test should be applied if located within Flood Zone 3a or that development should not be permitted within this area if located within Flood Zone 3b (functional flood plain).
7.2 The Exception Test
7.2.1 Paragraph 102 of the NPPF states that, if following a Sequential Test it is not possible for the development to be located in zones with a lower probability of flooding, An Exception Test can be applied.
7.2.2 The Exception Test, is a method to demonstrate and help ensure that flood risk to people and property will be managed satisfactorily, while allowing necessary development to go ahead in situations where suitable sites at lower risk of flooding are not available.
7.2.3 For an Exception Test to be passed it must be demonstrated that: The development provides wider sustainability benefits to the community that outweigh flood risk. The development will be safe for its lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall
7.2.4 Prior to the closure of both kilns approximately 500 people were directly employed full time in the Westbury Cement Works. Currently approximately 20 people are employed in the Tarmac distribution operations located in the western section of the Works. Up to 85 new fulltime jobs will be created following the development of the slab track manufacturing facility. In addition to these jobs, there is a potential for numerous employment opportunities to be created in the industries that support the operation of the facility. The
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development of the site will therefore provide a long-term economic benefit and a social boost to the local area.
7.2.5 The proposed development is situated within a large brownfield section of the Westbury Cement Works. The site has therefore a history of heavy industrial use, primarily cement production. The redevelopment of this existing brownfield site ensures that the environmental impact of the development is minimised as it is more sustainable to develop a brownfield site as opposed to a greenfield site.
7.2.6 The storage and logistics zones are a vital component of the precast manufacturing facility. Each area has been sized and located within the site to ensure the efficient operation of the facility. The eastern storage and logistics zone is situated between the precast manufacturing plant and the private rail siding. It is proposed that the finished track slabs will be stored within this storage area prior to their loading onto trains for dispatch throughout the UK via the rail network. The area and location of this storage and logistics zone is therefore confined to its present proposed location as no other suitable location are available.
7.2.7 The development of the site provides clear economic, social and environmental benefits to the local area. The majority of the site (91% of total area), is located within Flood Zone 1, with a smaller section (2% of total area) located within Flood Zone 2. As the development is classified as Less Vulnerable, the risk of flooding in these areas is considered low during a 1 in 100 year storm event. These areas are therefore considered to pass the Exception Test.
7.2.8 For the small section of the site (7% of total area) which is indicated on the Environment Agency’s strategic flood maps as being within Flood Zone 3 there is a potential risk of flooding during a 1 in 100 year flood event. This risk can be reduced through the introduction of suitable mitigation measures such as a combination of raising the ground levels in the developed area and providing flood storage compensation elsewhere to minimise the risk of flooding the site while not increasing the risk of flooding elsewhere.
7.2.9 To reduce the risk of pluvial flooding on the site the storm water drainage system will be designed to the appropriate standards. The application of sustainable drainage systems, such as storm water attenuation, to manage the discharge of water from the site will again minimise the risk of flooding within the site, while not increasing the risk of flooding elsewhere.
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7.2.10 The potential application of flood mitigation measures will be discussed further in Section 8 of the Report.
7.2.11 As the application of flood mitigation measures reduces the risk of flooding during a 1 in 100 year event to a low risk and as the development is considered to provide significant sustainable benefits it is considered that, when flood mitigation measures are provided, this section of the site also passes the Exception Test.
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8.0 FLOOD MITIGATION MEASURES
8.1 Fluvial flooding
8.1.1 Section 6 of this report identifies that the majority of the site is located within Flood Zone 1 and therefore the risk of flooding to these areas of the development are considered low. Within these sections of the site it is recommended that the finished floor level of all buildings and emergency access routes are maintained at a level of 600mm higher than the Q100 (+CC) flood level of the Heywood Brook at these locations.
8.1.2 Review of the Environment Agency recommended climate change allowance guidance would indicated that an allowance of between 20% and 25% should be applied. Confirmation on the required climate change allowance should be sought from the Environment Agency during detailed design.
8.1.3 Section 6 of this report identified a section of eastern storage and logistics area and the rail siding zone are located within Flood Zone 3 and are therefore considered to be at risk to flooding during a 1 in 100 year storm event.
8.1.4 It is estimated that the 1 in 100 year flood level is 61.25mOD in the affected area. (Note this estimation does not include allowance for climate change) Analysis of the estimated flood level against the existing ground levels indicates that the total volume of flood storage for this is approximately 850m3. This can be split into the following level bands:
Level band Volume of flood water on site (m3) 61.25 - 61.25mOD 560 61.05 - 61.15mOD 210 60.95 - 61.05mOD 75 60.85 - 61.95mOD 5 Table 8.1
8.1.5 Preliminary correspondence with the Environment Agency and with the Wiltshire Council indicates that for development to be permitted within areas designated as Flood Zone 3, suitably designed flood storage mitigation measures will be required. These measures, if considered required following the detailed modelling of the watercourse, will ensure the safety of the
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development and replace any flood storage volume lost as a result of the development.
8.1.6 A potential method of reducing the risk of flooding the identified area is by increasing the finished surface levels of the development to a level 600mm above that of the 1 in 100 year flood level of the Heywood Brook adjacent to this location. This would result in a minimum development level of approximately 61.85mOD.
8.1.7 If this flood mitigation measure is progressed in the detail design of the development then the displaced floodwater could be compensated for by creating flood storage compensation areas within the site. These storage areas could be created if appropriate by profiling the existing ground levels of areas adjacent to the Heywood Brook to provide an equal or greater storage volume capacity on a level for level basis. Table 8.2 details the volume to be provided at each level for the example option.
Level band Volume of Flood Storage Compensation provided (m3) 61.25 - 61.25mOD 560 61.05 - 61.15mOD 210 60.95 - 61.05mOD 75 60.85 - 61.95mOD 5 Table 8.2
8.1.8 An alternative option is that the ground levels in the affected area are maintained as per the existing levels and that the concrete slabs are placed onto supports to raise them above the Q100 (+CC) flood level. In this option the volume of floodwater storage removed is minimised with the only volume removed that of the support structures. The volume of flood storage compensation required would therefore be reduced accordingly.
8.1.9 The options described above are examples to provide indication at the outline planning application stage of the possible fluvial flood mitigation measures available and that are technically viable. It is recommended that further discussions are held with both the Environment Agency and Wiltshire Council during the detailed design of the development prior to the submission of the full planning application.
8.1.10 During the detailed design of the flood storage compensation measures the effects of climate change to the 1 in 100 year flood level should be considered
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and an appropriate allowance made for within the designed compensation measures.
8.2 Pluvial Flooding
8.2.1 Section 6 of this report identifies that a number of small, localised areas are identified as being at risk to pluvial flooding during a 1 in 100 year flood event. The design of the new sewerage systems will be undertaken to current design standards and therefore the risk of pluvial flooding to the development is considered low.
8.2.2 Currently storm water generated on the Westbury Cement Works (inclusive of the development site area) is discharged to a series of artificial ponds located in the east of the site. The water subsequently discharges into the clay pit and finally into the Heywood Brook. The current discharge consent held permits 850m3 of water per day to be discharged from the clay pit into the Brook. We would note that this is approximately 9l/s for the whole Westbury Cement Works area which is considered to be a very low rate given the total area is approximately 17ha.
8.2.3 At this outline design stage it is proposed that the storm water generated on the site will be discharged via the existing drainage arrangement into the Heywood Brook. The application of the existing artificial ponds and the large clay pit as a potential flood storage attenuation areas provides ample capacity within the drainage system to minimise pluvial flood risk.
8.2.4 The greenfield runoff rate for the Westbury Cement Works has been calculated to be 217l/s which equates to approximately 12.7l/s/ha (Appendix D). Wiltshire Council have commented that, for the purpose of development, greenfield runoff rate is considered to be 5l/s/ha. As the Westbury Cement Works is approximately 17ha and the proposed development area is 12.2ha this would equate to a greenfield runoff of 85l/s for the entire Works or 61l/s for the development area alone.
8.2.5 The proposed development area is considered to be 100% hard standing. The peak brownfield discharge rate generated on the development area during a 1 in 100 year return period, including an allowance for 20% climate change, is approximately 5,650l/s.
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8..2.6 Preliminary correspondence with the Wiltshire Council indicates that generally for a development to be permitted, the stormwater discharge rate from the site is to be reduced by a minimum of 20%.
8.2.7 As the both the brownfield and greenfield runoff rates are significantly higher than the existing Environment Agency discharge consent rate, the application of a 20% reduction to the existing discharge rate could be considered as overly cautious in this case.
8.2.8 The current drainage arrangement utilises diesel pumps to lift the water from the pit to its discharge point on the Heywood Brook. As this method of discharge relies on the mechanical operation of the pumps, there is a potential risk of flooding if the pumps fail and if this method is selected it is suggested that back up resilience could be provided. We would note though that due to the large volumetric capacity of the clay pit this risk is considered low
8.2.9 Consideration of a gravity only storm sewerage system should be undertaken during the detailed design stage as this would remove any risk of flooding resulting from the failure of the mechanical aspects such as pumps. Of note however is that the attenuation capacity provided by the clay pit may not be usable in a gravity system.
8.2.10 An example case of a gravity only system is the development of the three ponds into an cascade attenuation system discharging directly into the Heywood Brook with the discharge rate limited to that equal to the greenfield runoff for the Works. The three ponds have a total surface area of approximately 4,900m2. Modification of the ponds (if appropriate) and the application of a flow control device such as a Hydrobrake or Q-Brake could create an attenuation system capable of managing the surface water run off generated on the development site.
8.2.11 Using the Wiltshire Council recommended discharge rate of 85l/s as an example the attenuation system would require a maximum storage capacity of approximately 10,000m3. If the ponds are used this would equate to approximately 2.0m depth of water over their 4,900m2 surface area. (see appendix D for stormwater attenuation calculation).
8.2.12 Alternatively if the 12l/s/ha greenfield rate was used then the maximum storage capacity required would be approximately 7,800m3. This would equate to approximately 1.6m depth of water over the three ponds 4,900m2 surface area. (see appendix D for stormwater attenuation calculation).
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8.2.13 Due to the low permeability of the superficial deposits and underlying bedrock on the site, infiltration of stormwater through soakaways is not considered to be a viable option at this stage. It is recommended that during the detailed design of the development permeability testing is undertaken to inform the design of the SuDS system.
8.2.14 The above examples are only to provide an indication at the outline planning application stage of the possible pluvial flood mitigation measures available and that are technically viable. It is recommended that further discussions are held with both the Environment Agency and Wiltshire Council during the detailed design of the development prior to submission of the full planning application.
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9.0 CONCLUSION
9.1 This report has been prepared by Doran Consulting to accompany an outline planning application for the development.
9.2 The proposed facility will manufacture slab tracks to be used in the UK rail network. It is expected that up to 85 people directly employed fulltime by the facility when fully operational.
9.3 The site is located within the Westbury Cement Works, approximately 2 km northeast of Wesbury Town, County Wiltshire. The facility will be located within the recently demolished northern and eastern sections of the Westbury Cement Works. The total area of the development site is approximately 12.2ha.
9.4 The proposed facility will consist of a large slab track manufacturing facility and supporting storage and logistics areas where the formed precast concrete slab track units will be stored following production. The facility will utilise the existing rail connection to transport the finished precast units by train throughout the UK.
9.5 At this stage of the design process, a reasonable approach to assessing the potential risk of flooding to the development is through the review of the Environment Agency’s Long term strategic flood mapping.
9.6 The Flood zones identified on the Environment Agency’s maps are intended to be for strategic purposes only and as such are indicative and are not considered to be site specific.
9.7 For detailed site specific information, It is recommended that modelling of the Heywood Brook using topographical survey data of the watercourse, development site and surrounding area is undertaken during the detailed design of the development to inform the full planning application.
9.8 During the 57 year operational history of the site there has been no recorded instances of flooding occurring with any areas of the Works. Review of the Flood Risk Assessment Slab Track Manufacturing Facility, Westbury, Co. Wiltshire 26
Environment Agency’s historical flood mapping in relation to the proposed development indicates that there have been no recorded instances of flooding at the site.
9.9 Review of the Environment Agency’s long term fluvial flood risk mapping indicates that a section of the eastern storage and logistics zone and a small section of the rail siding zone are located within Flood Zone 3. These areas are therefore considered to be at risk to flooding during a 1 in 100 year flood event. The total area of the development site indicated as within Food Zone 3 is approximately 8,600m2 (7% of the development area).
9.10 Review of the Environment Agency’s long term pluvial flood risk mapping indicates that the risk of pluvial flooding during a 1 in 100 year flood event is low for 98% of the development site area. The combined area of the development site indicated to be at risk is approximately 2,320m2 (2% of development area).
9.11 Due to the elevation of the development site and its distance from the coast the risk of coastal flooding to the site is considered to be low.
9.12 Review of the Environment Agency long term reservoir inundation flood risk map for the area indicates that the development site is not at risk to flooding in the event of an uncontrolled release of water from the reservoir located at Long Mead Wood.
9.13 Review of the British Geological Society hydrogeology records for the site state that the bedrock is classified as mudstone with essentially no ground water. The overlying superficial deposits are Head material comprising of clay with silts, sand and gravels. Due to the low permeability of the strata the risk to the site from ground water flooding is considered to be low.
9.14 As the proposed development will be served by sewer networks designed to current standards it is considered that the risk of out of sewer flooding to the development site is low.
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9.15 Application of the Sequential Test determines that the majority of the site (93%) is considered acceptable for development.
9.16 Application of the Exception Test to the remaining 7% of the site identifies the clear economic, social and environmental benefits to the local area. In addition the application of flood mitigation measures to this area can reduce the risk of flooding on the site without increasing the risk of flooding elsewhere. This area of the site is therefore considered to pass both parts of the Exception Test
9.17 It is advised that the finished floor levels of all buildings and emergency access routes of the proposed development are set at a minimum of 600m above the 1 in 100 year (plus climate change) fluvial flood level of the Heywood Brook.
9.18 It is estimated that the 1 in 100 year flood level in the area of the development located within Flood Zone 3 is 61.25mOD. (note this estimation does not include allowance for climate change). A potential method of reducing the risk of fluvial flooding in this area is by increasing the finished surface levels of the development to a level 600mm above that of the Q100 (+CC) flood level. This would result in a minimum development level in this area of approximately 61.85mOD.
9.19 If this mitigation option is progressed in the detail design, following the outcome of the hydraulic modelling of Heywood Brook, the displaced floodwater could be compensated for by creating flood storage compensation areas within the site. These storage areas could be created by profiling the existing ground levels of areas adjacent to the Heywood Brook to provide an equal or greater storage volume capacity on a level for level basis.
9.20 Preliminary correspondence with the Wiltshire Council indicates that for development to be permitted the stormwater discharge rate from the site is to be reduced by a minimum of 20%. This reduction can be achieved through the application of sustainable drainage design such as attenuation systems.
9.21 As the both the brownfield and greenfield runoff rates are significantly higher than the existing Environment Agency discharge consent rate, the application of
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a 20% reduction to the existing discharge rate could be considered as overly cautious in this case.
9.22 At this outline design stage it is proposed that the storm water generated on the site will be discharged via the existing drainage arrangement into the Heywood Brook. The application of the existing artificial ponds and the large clay pit as a potential flood storage attenuation area provides ample capacity within the drainage system to minimise pluvial flood risk.
9.23 The current drainage arrangement utilises diesel pumps to lift the water from the pit to its discharge point on the Heywood Brook. As this method of discharge relies on the mechanical operation of the pumps, there is a potential risk of flooding to the area local to the clay pit if the pumps fail and if this method is selected it is suggested that back up resilience could be provided. We would note though that due to the large volumetric capacity of the clay pit this risk is considered low.
9.24 Consideration of a gravity only storm sewerage system should be undertaken during the detailed design stage as this would remove any risk of flooding resulting from the failure of the mechanical aspects such as pumps. Of note however is that the attenuation capacity provided by the clay pit may not be usable in a gravity system.
9.25 Due to the low permeability of the superficial deposits and underlying bedrock on the site, infiltration of stormwater through soakaways is not considered to be a viable option at this stage. It is recommended that during the detailed design of the development permeability testing is undertaken to inform the design of the SuDS system.
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9.26 The example flood mitigation measures within this report provide an indication at the outline planning application stage of the possible fluvial and pluvial flood mitigation measures available and that are technically viable. It is recommended that further discussions are held with both the Environment Agency and Wiltshire Council during the detailed design of the development prior to the submission of the full planning application.
Campbell Davis Philip McManus Director Graduate Doran Consulting Doran Consulting Our ref: pmm/172056A/cd
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